Improvements in or relating to dewatering apparatus

Abstract

 Apparatus for dewatering a fibrous suspension on a paper forming machine comprises a suction housing (7) having a convexly curved active surface (9) adapted to contact a travelling forming wire (1) of the paper machine. Suction and pressure is applied to the fibrous suspension over the length of the active surface (9) for the purpose of dewatering the suspension.
In order to increase the drainage capabilities of the dewatering apparatus the upstream radius (R1) of curvature of the active surface (9) is arranged to be greater than the downstream radius (R3) of curvature to thereby increase the pressure applied to the web over the length of the active surface. In one embodiment the active surface has three discrete radii R₁, Rs, R₃ in which R₁>R₂>R₃. In another embodiment the radius increases progressively.

Description

[0001] The present invention relates to apparatus for dewatering a fibrous stock and in particular for dewatering a fibrous web formed between top and bottom wires of a paper forming machine.

[0002] In a known paper forming machine a web is formed between top and bottom forming wires and water is extracted in an upward direction by suction by a so- called inverted vacuum box. The vacuum box has a cover whose surface contacting the top wire is open to vacuum. The openings in the cover are in the form of slots and/or drilled openings. Typically, the first part of the cover after the auto-slice has slots and a later part has holes. In a known construction the cover has a constant radius of curvature.

[0003] An aim of the present invention is to increase the drainage capabilities-of the curved inverted vacuum box without detracting from its-paper-forming qualities.

[0004] According'to one aspect of the present invention there is provided apparatus for dewatering a fibrous suspension on a paper forming machine comprising a suction housing having an active surface adapted to contact a travelling forming wire of the paper machine, said active surface being curved convexly, with the radius of curvature of the active surface upstream being greater than the radius of curvature downstream.

[0005] In a preferred embodiment the radius.of curvature of the cover decreases progressively in the downstream direction, i.e. in the direction of travel of the wire. Thus, the cover has a compound curvature. As an alternative, the cover may be composed of two or more constant radially curved sections, having different radii.

[0006] The suction housing is preferably an inverted suction box in which water is extracted upwardly from the web which is constrained between top and bottom forming wires. When the stock consistency is low, on entry between the top and bottom forming wires, the large radius contributes to low pressure between the two wires, and the stock is dewatered gently. As the stock between the wires increases in consistency due to the dewatering action, higher pressures can be applied to the stock without having a crushing effect, and more water can be dewatered from it.

[0007] According to another aspect of the present invention there is provided a method of dewatering a fibrous suspension on a paper forming machine comprising applying suction and pressure to the fibrous suspension by contacting a travelling forming wire of the paper forming machine with a convexly curved active surface of a suction housing wherein the pressure exerted on the fibrous suspension over the length of the active surface is greater downstream than upstream.

[0008] The pressure is inversely proportional to the radius of the active surface and may increase incrementally or progressively over the length of the active surface.

[0009] The initial large radius gives the same high quality formation of the present large radius, but has the advantage that the higher pressures applied later in the dewatering process increase the drainage capabilities of the cover.

[0010] The present invention will now be described further, by way of example only, with reference to the accompanying drawing, which illustrates a'diagrammatic side view of dewatering apparatus on a paper forming machine.

[0011] Referring to the drawing, an endless top wire 1 is entrained around rollers 3 and is co-operable with a bottom forming wire 5 of a paper forming machine. The wires 5 and 1 move in the direction indicated by the respective arrows A and B. The fibrous suspension is deposited on the forming wire 5. Suction apparatus in the form of a curved inverted vacuum box 7 is disposed within the endless top wire 1 and has an active surface 9 which contacts the top wire 1. The active surface 9 is open to vacuum and may comprise a plurality of bars defining slots, or holes drilled in a plate. Thus, the suspension is dewatered upwardly in the illustrated embodiment. The active surface is curved convexly and according to one aspect of the present invention the radius of curvature of the active surface or cover is not constant throughout but decreases in the direction of wire movement. Preferably, the radius of curvature decreases progressively in the downstream direction. In practice; the cover may have two or more curvatures which are themselves of constant radius but which are different from one another. -Thus, in the illustrated embodiment the cover may have three discrete curvatures, R₁, R₂ and R₃, with R₁>R₂>R₃. For example, ^R₁ ⁼ 5m, R_{2 =} 3m and R₃ = 2m. A lead in foil 11 is provided, and a transfer box 13.

[0012] Ideally, the radii will be tangent to each other to define a compound curvature composed of decreasing radii.

[0013] Theoretically, the pressure between the two wires .1, 5 on the curved inverted suction box is inversely proportional to the radius of curvature. The progressively decreasing radius of curvature will have the effect of progressively increasing the pressure exerted by the wires on the stock between them. For example, if the starting radius is 5 metres, the pressure P₅ between the wires will be P_s = k x

where k is a constant. If the following part of the curvature has a radius of 2.5m, the pressure that the stock will be subjected to will be 2.5 times the initial pressure, as can be seen from the relationship:-

[0014] In addition to the above mentioned advantages with the cover of the present invention the bone dry moisture of the stock, after the compound cover, will be higher than from a cover with uniform radius. Additionally, due to the increased drainage capacity, more water will flow through the cover -apertures and this increased flow flushes the internal passageways and clears up all stock build-ups. Thus, the.tendency for the final part of the cover to plug up is reduced.

Claims

1. Apparatus for dewatering a fibrous suspension on a paper forming machine comprising a suction housing (7) having an active surface (9) adapted to contact a travelling forming wire of the paper machine, said active surface being curved convexly and characterised in that the radius of curvature of the active surface upstream is greater than the radius of curvature downstream.

2. Apparatus as claimed in claim 1 in which the convexly curved active surface is composed of two or more constant radius curved sections having different radii.

3. Apparatus as claimed in claims 1 or 2 in which the active surface has three discrete curved sections having radii R₁, R₂, R3 and in which R₁>R₂>R₃.

4. Apparatus as claimed in claims 2 or 3 in which the radii are tangent to each other to define a compound curvature composed of sections of decreasing radius.

5. Apparatus as claimed in claim 1 in which radius of curvature of the active surface decreases progressively in the downstream direction.

6. Apparatus for dewatering a fibrous suspension as claimed in any preceding claim in which the fibrous suspension is disposed between top ardbottom forming wires and the top forming wire contacts the active surface and the active surface forms part of an inverted suction box whereby water is extracted upwardly from the fibrous suspension.

7. A method of dewatering a fibrous suspension on a paper forming machine comprising applying suction -and pressure to the fibrous suspension by contacting a travelling forming wire of the paper forming machine with a convexly curved active surface (9) of a suction housing(7),characterised in that the pressure exerted on the fibrous suspension over the length of the active surface is greater downstream than upstream.

8. A method as claimed in claim 7 in which the pressure increases in discrete increments in inverse proportion to the radius of curvature of the active surface.

9. A method as claimed in claim 7 in which the pressure increases progressively between the upstream and downstream ends of the active surface.

Drawing